As for the oil, that comp is probably hermetic piston compressor, right? They come precharged from the factory according to refrigerant they are meant to be used with. So the way to find the oil type would be to contact comp manufacturer... 27 years old window unit, ouch... But even if you would want to replace the oil, I still think it would be more cost effective to buy brand new window unit, let this run as it is for as long it will run...